This invention is in the field of radiolabeled imaging and radioactive therapy agents. In particular, derivatives of urea based prostate-specific membrane antigen (PSMA) inhibitors are disclosed. Derivatives with a chelating moiety are capable of chelating a radioactive metal. Compounds containing a novel phenoxy linker were also prepared, and the linker attaches a chelating moiety or a radioactive group with an urea based PSMA targeting moiety.
Prostate-specific membrane antigen is a highly specific prostate epithelial cell membrane antigen. It is a type II transmembrane protein consisting of a short NH2-terminal cytoplasmic domain, hydrophobic transmembrane region, and a large extracellular domain. This is a transmembrane enzyme with overlapping carboxypeptidase enzyme activities similar to (a) glutamate carboxypeptidase II (GCPII, E.C.3.17.21), a zinc-dependent metallopeptidase, and (b) folylpolyglutamate synthetase (FPGS). The extracellular portion of the peptide sequence exists as a dimer and shows a strong binding to glutamate and glutamate related structures (brain related PSMA), its natural substrates are N-acetyl-aspartylglutamate and folyl-poly-γ-glutamates (prostate related PSMA) (Scheme 1).

PSMA is highly expressed in various tumors, including prostate cancer. Often, PSMA expression increases in higher-grade cancers and metastatic diseases. In the vast majority of neovasculature in solid tumors, there is high expression of PSMA, but not in normal vasculature. This makes PSMA a suitable target for cancer detection and therapy. Prostascint® (In-111 Capromab pendetide) developed by Cytogen was the first antibody of PSMA approved for clinical use. This antibody only recognizes the intracellular epitope on PSMA, which is associated with dead or necrotic cells commonly found in lymph nodes. Prostascint® is not useful for imaging living tumor cells because of its lack of cell penetration. SPECT (single photon emission computer tomography) imaging of this agent exhibits prolonged background activity and an unfavorable signal to background ratio even at 4 days post injection.
A specific antibody targeting the extracellular portion of PSMA, J591, has been reported and shown to have improved PSMA targeting properties. This antibody has been radiolabeled with various isotopes, 89Zr, 111In, 177Lu, etc., for imaging and radiotherapy. J591 is an antibody against the extracellular epitope of PSMA, and it is targeting the PSMA binding sites on the membrane of tumor cells. Its in vivo retention and circulation time is relatively long, thus contributing to a prolonged waiting period to reach optimal imaging. An isotope with a longer physical half-life is essential for this purpose, therefore 89Zr, a positron-emitting isotope with a physical half-life of 78.4 hours, is more appropriate. [89Zr]J591 bound strongly to PSMA, and clinical studies in humans suggested that it is useful for defining the tumor location by PET imaging.
A number of small molecule-based PSMA imaging agents have been reported in the literature. Different PSMA-targeting core structures have been employed, including: 2[(3-amino-3-carboxypropyl)(hydroxy)(phosphinyl)-methyl]pentane-1,5-dioic acid (GPI), 2-(3-mercaptopropyl)pentane-dioic acid (2-PMPA), phosphoramidates, and urea (Glu-NH—CO—NH-Lys(Ahx)), originally reported in 2000 (Scheme 2). See e.g. US2004054190; Kozikowski A P, et al., J. Med. Chem. 47:1729-38 (2004). Based on these binding core structures, many of the PSMA inhibitors were reported to be highly selective and potent. After labeling with different isotopes, they can be employed for in vivo imaging (SPECT or PET).

Several potential PSMA-targeted imaging agents using urea based ligand systems (Glu-NH—CO—NH or Glu-NH—CO—NH-Lys(Ahx)), including SPECT imaging agents: [123I]MIP-1072, [123I]MIP-1095 [49-51], [99mTc]MIP-1404, and [99mTc]Tc-MIP-1405 (Scheme 3), have entered into clinical trials. Results of phase II clinical studies suggest that these SPECT PSMA imaging agents are suitable for the diagnosis of prostate and other related solid tumors.

Several 11C and 18F labeled PET imaging agents targeting PSMA have also been reported (Scheme 4). Again, these are derivatives of Glu-NH—CO—NH— or Glu-NH—CO—NH-Lys(Ahx), such as [11C](S)-2-[3-((R)-1-carboxy-2-methylsulfanyl-ethyl)-ureido]-pentanedioic acid, 11C-MCG, Two fluorinated version of PSMA-targeting agents, [18F]DCFBC: N—[N—[(S)-1,3-dicarboxypropyl]carbamoyl]-4-[18F]-fluorobenzyl-L-cysteine, and [18F]DCFPyL: 2-(3-(1-carboxy-5-[(6-[18]fluoro-pyridine-3-carbonyl)-amino]-pentyl)-ureido)-pentanedioic acid, have been reported. Both agents showed promising results in imaging patients with metastatic prostate cancer. The preparation of 11C and 18F labeled PSMA imaging agents require a near-by cyclotron, because the physical half-life is 20 min and 110 min, respectively. As an alternative, 68Ga can be used for PET imaging in a laboratory setting without a near-by cyclotron.

In the past few years, [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer, [68Ga]1a) and its dimer, [68Ga](Glu-NH—CO—NH-Lys(Ahx))2-HBED-CC were successfully prepared and showed high PSMA binding (Scheme 5). Although both [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC (monomer) and [68Ga](Glu-NH—CO—NH-Lys(Ahx))2-HBED-CC (dimer) exhibited comparable preclinical data, currently, the most popular PSMA/PET imaging agent that has been successfully applied in humans is [68Ga]Glu-NH—CO—NH-Lys(Ahx)-HBED-CC. See Eder M, et al., Bioconjug. Chem. 23:688-97 (2012).

Recently PSMA-617 and DOTAGA-(yl)-fk(sub-KuE) (I&T) were reported (Scheme 6). These two compounds contain different linkers between the chelating moiety and the urea based PSMA targeting moiety. These linkers have various amino acid residues. These PET tracers appear to provide useful diagnostic information in humans. A comparison of PET imaging using [68Ga]Ga-PSMA-HBED-CC and [18F]DCFPyL, in prostate cancer patients has been reported. Additional imaging agents with structure modifications in the linker regions have been reported to have improved tumor targeting properties and pharmacokinetics. See US Published Appl. No. 2016/0228587.

A need continued to exist to further improve the Glu-NH—CO—NH-Lys(Ahx)-HBED-CC amide derivatives as PSMA inhibitor for in vivo imaging and radiation therapy.